This invention generally relates to a mobile telephone communication system and more particularly to such a system with facilities for handling emergency calls.
Apparatus and methods have been previously proposed for determining the location of a mobile telephone upon receipt of an emergency call from the mobile telephone and otherwise.
A difficulty with these systems is that they require a modification of the infrastructure of the communication network within which they operate. Also, some of the proposed locating system will not work with both satellite communication systems and terrestrial based cellular communication networks. Moreover, these systems disadvantageously require rescue personnel to be located at the emergency center to receive the positional information which may or may not be accurate.
There is currently a system used widely by hunters, the merchant marine and boaters which makes use of emergency radio beacons broadcast on an internationally agreed channel. While this beacon system allows tracking of, for instance, a boater in distress, it is not integrated with an emergency call handling system or mobile telephone. Also, the emergency beacon transmission is initiated from the remote radio transmitter which can result in inadvertent actuation when, in fact, there is no emergency. There have been cases where accidental activation of the beacon has resulted in wasteful and unneeded searches.
COSPAS-SARSAT is an international satellite system for Search and Rescue. It consists of a constellation of four satellites in polar orbit and a network of earth stations which provide distress alert and location information to appropriate rescue authorities anywhere in the world, for maritime, aviation and land users in distress. In addition, the U.S., French, Canadian and other governments provide two geo-stationary satellites over North America. The frequencies used include 121.5 MHz, 243 MHz and 406.5 MHz.
ICO Global Communications is providing a global mobile satellite system which uses a system of satellites to communicate with the user handsets. Other mobile satellite systems include those of Irridium and Globalstar. In addition, there are a number of regional satellite systems which make use of one or more geosynchronous satellites. The system from ICO Global Communications uses a Time Division Multiple Access (TDMA) air interface and a Global System for Mobile Communication (GSM) network architecture.
In the ICO Global Communications network, it is possible to place emergency calls as would be done in a normal cellular network. One difference is that determining the position of the user in distress is problematic due to the large coverage area and position determination techniques. Generally, when a subscriber places an emergency call, a call is set up to an emergency answering center.
A number of techniques are known for delivering "cell" or service area information to an emergency center. However, in one case, a calling party number field is populated with the cell identity. In the case of the ICO Global Communication network, a service area can as large as an entire country. There is currently no means to transfer actual positional information from the mobile unit within the service area or cell to the emergency center, only the cell or service area itself can be transferred.
However, it has been proposed to have the mobile unit send a set of coordinates to the emergency center. The accuracy of these coordinates, in the theoretical case that they could be delivered, are dependent on the accuracy of the information used to perform the position calculation. In the case where the position is calculated using the Global Positioning System, the coordinates are quite accurate indeed. However, in the case where the coordinates are calculated based on delay or power measurements, the accuracy can vary depending on a number of factors, including the distance from the center of the satellite spot beam. Inaccuracy in positioning location on the order of scores of kilometers based on radio measurements are possibly introduced.
In either case, position updates are not available during the course of a call, but are only available when communication between the terminal and the network is first established.